1. Field of the Invention
The invention relates to a synchronization signal decoder and associated method.
2. Description of the Related Art
The National Television System Committee (NTSC) is responsible for setting television and video standards in the United States and other parts of the world. The NTSC television standard defines a composite video signal with a refresh rate of 60 half frames (interlaced) per second. Each frame contains 525 lines and up to 16 million different colors. The standard uses only about 480 of these lines to transmit video information. It uses the additional 45 lines to carry control codes (such as interlace information), closed captions, and other similar non-video content. Many companies add distortion or scrambling signals to some or all of these 45 additional lines to prevent unauthorized copying of copyright protected works. One such company is Macrovision® Inc., a Delaware corporation, whose protection signals carry the same name.
Macrovision® copy prevention works by adding predetermined synchronization and content signals (collectively distortion signals) to these additional lines prior to mass distribution. Automatic Gain Control (AGC) circuits included in most recording equipment scramble the video signal responsive to the distortion signals when a user attempts to copy protected media. Recording equipment includes videocassette recorders (VCRs) and the like. Images copied without authorization from Macrovision®-encoded source material will frequently exhibit image distortion including color loss, image tearing, variable brightness, and picture instability.
Put differently, the additional distortion signals are designed to make recording equipment, such as a VCR, malfunction if it attempts to record protected material. Recording equipment typically locks on to the incoming video's synchronization signals to ensure proper alignment. But the distortion signals include about 32 “illegal” synchronization signals in the non-viewable area of the picture that confuse the equipment and result in a misaligned and distorted image.
Distortion signals do not interfere with most older televisions that are capable of distinguishing distortion signals from other synchronization signals. But more sophisticated, typically newer, televisions and other digital pixelated displays—e.g., liquid crystal display (LCD) projector, flat panel monitor, plasma display (PDP), field emissive display (FED), electro-luminescent (EL) display, micro-mirror technology display, and the like—often malfunction because of the distortion signals. This is in part due to technology in newer sets and displays that use synchronization signals to further process the images to be displayed. If the technology is unable to parse the distortion signals from the incoming synchronization signals, the image will not be properly display on the set's screen. For properly displaying and processing images, then, the issue is distinguishing the distortion signals from other synchronization signals in a composite video signal stream without causing unacceptable timing deterioration or loss of logical meaning.
Previous attempts at distortion signal parsing center on gating techniques inserted into the signal stream. These gating techniques operate to alter the timing of the signals and often result in greater circuit complexity. Other approaches include using phase locked loops that are susceptible to noise and require precise and expensive components
Accordingly, a need remains for an improved circuit capable of distinguishing distortion signals other from synchronization signals in image processing applications.